Stress-corrosion-resistant alloy

ABSTRACT

An austenitic stainless alloy which is resistant to stresscorrosion cracking containing 18-23 atomic percent chromium, 1218 atomic percent nickel, and 0.2-1.5 atomic percent beryllium, the amount of nitrogen being limited to not greater than 200 parts per million. The alloy may also include 0.5-3.0 atomic percent aluminum and/or 0.05-0.2 atomic percent carbon.

United States Patent inventors Roger W. Staehle Columbus, Ohio; Juan Royuela, Madrid, Spain Appl. No. 781,802

Filed 1 Dec. 6, 1968 Patented Nov. 9, 1971 Assignee The United States of America represented by the United States Atomic Energy Commission STRESS-CORROSION-RESISTANT ALLOY [56] References Cited UNITED STATES PATENTS 2,029,724 2/1936 Krull 75/123 C 2,482,097 9/1949 Clarke.. 75/123 C 2,590,074 3/1952 Bloom 75/128 Primary lixaminer-Hyland Bizot Atlomey-Roland A. Anderson greater than 200 parts per million. The alloy may also include 0.5-3.0 atomic percent aluminum and/or 0.050.2 atomic percent carbon.

7 Claims, 3 Drawing Figs.

US. Cl 75/124, 75/ 128 R Int. Cl ..C22c 37/10, C22c 39/02 Field of Search 75/123 BE, 126 G, 128

80 E k 60 3 It 40 S 0 a 2 I I0 TIME, M/IVU r55 STRESS-CORROSION-RESISTANT ALLOY CONTRACTUAL ORIGIN OF THE INVENTION The invention described herein was made in the course of, or under, a contract with the UNITED STATES ATOMIC ENERGY COMMISSION.

BACKGROUND OF THE INVENTION This invention relates to alloys such as chromium-nickel stainless steels which are resistant to stress-corrosion cracking. As is pointed out in the Metals Handbook, 1961 Edition, on page 566, almost any metal or alloy, including austenitic stainless steels, can be made to fail by stress-corrosion cracking under conditions involving applied or residual stresses and specific mild corrosives. Chloride solutions are the worst offenders in promoting cracking of austenitic stainless steels, and a number of failures involving chloride compounds have occurred in industry. Naturally, therefore, there have been many proposals for overcoming the problem; none of these proposals have resulted in a completely satisfactory solution to the problem.

SUMMARY OF THE INVENTION The invention accordingly comprises a new austenitic stainless steel which is more resistant to stress-corrosion cracking than are other comparable alloys. The alloy includes 18-23 a/o (atomic percent) chromium, 12-18 a/o nickel, and 0.2-1.5 alo beryllium, the amount of nitrogen being limited to 200 parts per million. The alloy may also include 0.5 to 3.0 a/o aluminum and/or 0.05-0.2 a/o carbon.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a graph containing a family of curves showing the effect on the stress-corrosion resistance of a base alloy containing 65 alo iron, 15 a/o nickel and 20 alo chromium of the addition of several different percentages of beryllium thereto.

FIG. 2 is a similar graph showing the effect on the stresscorrosion resistance of the said base alloy which contains 1.25 a/o beryllium of the addition of several other constituents thereto.

FIG. 3 is a similar graph showing the effect on the stresscorrosion resistance of several standard stainless steels of the addition of beryllium thereto.

DESCRIPTION OF THE PREFERRED EMBODIMENT Results plotted in the curves included in the drawing were obtained, in accordance with usual practice, by testing samples in boiling MgCl, These tests were performed by attaching one end of a test wire 0.015 inch in diameter to the bottom of a container holding an aqueous MgCl, solution 45 percent MgCl,; 55 percent H,0--B.P. 154 C.), placing the test wire under stress by attaching the free end of the wire to a nylon cord which passes over two pulleys and is attached to a weight. The MgCl solution is boiled and the time to failure of the wire is determined. For commercial alloys this time may be no longer than or minutes; according to the present invention, the time may be 10,000 minutes or over. Accordingly, an automatic timer is used to record the time of failure of the test wire.

The weight used is a shot-filled bottle and the weight of shot used is that amount calculated to place the test wire under a stress equal to 90 percent of the 0.2 percent offset yield stress of the alloy.

The test wires were prepared from high-purity materials which were vacuum-melted twice to remove nitrogen and thus these tests are not demonstrative of the behavior commercial alloys including the same constituents, since it is well known that stress-corrosion resistance of austenitic stainless steels can be improved by increasing the purity of the alloy. However, the results shown indicate a substantial improvement over the best results heretofore attained by increasing the purity.

Advantageous results attained by alloying iron-15 percent nickel20 percent chromium stainless steel with beryllium are clearly shown in FIG. 1. Alloys containing 0.1 a/o, 0.5 a/o, and 1.0 a/o beryllium were appreciably better than was the base alloy. Thus only 20 percent of the wires formed of the base alloy were unbroken after 7,500 minutes, whereas 40 percent or more of the test wires containing 0.1, 0.5, and 1.0 alo beryllium were unbroken after 20,000 minutes. It is also readily apparent that alloys containing 2.0 or 4.0 a/o beryllium were less satisfactory, since only 20 percent of the 4.0 alo alloys were unbroken at 11,000 minutes and the same percentage of 2.0 alo alloys were unbroken after 16,000 minutes. It is believed that maximum stress-corrosion resistance is attained in an alloy containing about 15 percent nickel and 20 percent chromium and between 0.1 and 1.5 a/o beryllium. These and other tests also indicate that the proportion of nickel may vary between 12 and 18 a/o and the proportion of chromium between 18 and 23 a/o while retaining the benefits of the present invention.

Similarly, FIG. 2 shows the advantageous results attained by adding a small quantity of carbon or aluminum to the beryllium-containing alloys and also, for comparison, the effect of adding silicon, which is known to improve the stress-corrosion resistance of austenitic stainless steel.

These and other tests indicate that the amount of carbon to be added should be between 0.05 and 0.2 a/o and the amount of aluminum between 0.5 and 3.0 a/o. By analogy, it is believed that both carbon and aluminum in the amounts specified may be included in the alloy.

The above tests were all made with alloys that were low in nitrogen-50 to 200 p.p.m.-and it is essential that the amount of nitrogen be held at a low level to attain the benefits of the present invention. While it is known that a reduction in the amount of nitrogen present in an austenitic stainless steel will increase the stress-corrosion resistance of the alloy, it was not known heretofore that even better results could be attained by adding beryllium to an alloy low in nitrogen and containing nickel and chromium within a relatively narrow range. As was known to the prior art, the length of time to breaking is improved up to a factor of 10 by reducing the proportion of nitrogen present. In accordance with this invention, beryllium addition gives at least an added factor of five improvement, while beryllium plus carbon or beryllium plus aluminum gives an additional factor of two. As is shown in FIG. 3, beryllium added to normal high-nitrogen stainless steel (Type 304 contains iron plus 8-12 percent nickel plus 18-20 percent chromium and Type 309 contains iron plus 12-15 percent nickel plus 22-24 percent chromium) gives only about a factor of two improvement up to less than 1,000 minutes to breaking.

It is believed that the improvement attained in accordance with the present invention is due to a chemical effect rather than a physical effect-alloys containing beryllium are not as reactive chemically as are other stainless steel alloys. This may be due to one addition slowing down reduction reactions, while another one slows down oxidation reactions.

It will be understood that the invention should not be limited to the details given herein but that it may be modified within the scope of the appended claims.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An alloy consisting of 20 atom percent chromium, 15 atom percent nickel, 1 atom percent beryllium, and less than 200 p.p.m. nitrogen, the balance being iron.

2. An alloy consisting of 18-23 atom percent chromium, 12-18 atom percent nickel, 0.1-1.5 atom percent beryllium, 0.5-3.0 atom percent aluminum, and less than 200 p.p.m. nitrogen, the balance being iron.

3. vAn alloy according to claim 2 consisting of 20 atom percent chromium, 15 atom percent nickel, 1.25 atom percent beryllium, 1.0 atom percent aluminum, and less than 200 p.p.m. nitrogen, the balance being iron.

um, l21 8 atom percent nickel, 0.1-1.5 atom percent beryllium, 0.5-3.0 atom percent aluminum, 0.050.2 atom percent carbon, and less than 200 p.p.m. nitrogen, the balance being tron.

7. Stainless steel according to claim 6 consisting of 20 atom percent chromium, l5 atom percent nickel, 1.25 atom percent beryllium, 1.0 atom percent aluminum, 0.1 atompercent carbon, and less than 200 p.p.m. nitrogen, the balance being iron.

t i i I #0 

2. An alloy consisting of 18-23 atom percent chromium, 12-18 atom percent nickel, 0.1-1.5 atom percent beryllium, 0.5-3.0 atom percent aluminum, and less than 200 p.p.m. nitrogen, the balance being iron.
 3. An alloy according to claim 2 consisting of 20 atom percent chromium, 15 atom percent nickel, 1.25 atom percent beryllium, 1.0 atom percent aluminum, and less than 200 p.p.m. nitrogen, the balance being iron.
 4. Stainless steel consisting of 18-23 atom percent chromium, 12-18 atom percent nickel, 0.1-1.5 atom percent beryllium, 0.05-0.2 atom percent carbon and less than 200 p.p.m. nitrogen, the balance being iron.
 5. Stainless steel according to claim 4 consisting of 20 atom percent chromium, 15 atom percent nickel, 1.25 atom percent beryllium, 0.1 atom percent carbon, and less than 200 p.p.m. nitrogen, the balance being iron.
 6. Stainless steel consisting of 18-23 atom percent chromium, 12-18 atom percent nickel, 0.1-1.5 atom percent beryllium, 0.5-3.0 atom percent aluminum, 0.05-0.2 atom percent carbon, and less than 200 p.p.m. nitrogen, the balance being iron.
 7. Stainless steel according to claim 6 consisting of 20 atom percent chromium, 15 atom percent nickel, 1.25 atom percent beryllium, 1.0 atom percent aluminum, 0.1 atom percent carbon, and less than 200 p.p.m. nitrogen, the balance being iron. 